The variance provides a description of how spread out the data you
collected is. The variance is computed as the average squared
deviation of each number from its mean. Mathematically speaking the
formula looks like this:
Where M is equal to the mean of the data, and N is equal to the
number of data points.
Example
To better show you how to calculate the variance we will now go over
an example. Let’s assume the following 4 data points represent trees per acre
of our sample points for a potential treatment area.
156, 176, 184, 209
We would all agree that the range of this data is 53 trees per
acre, and the mean is equal to about 181 trees per acre. We now need
to calculate the variance. You can begin by subtracting the mean
from each value and then squaring the result as shown below:
(156 – 181)2 = 625
(176 – 181)2 = 25
(184 - 181)2 = 9
(209 - 181)2 = 784
The next step is to add together the results. You should get 1443
Last you divide this number by the number of sample points minus 1.
You should end up with the following solution:
S2 = 481
Calculating the variance is important for many other statistical
calculations. So it is important that you have a basic understanding
of how this value is calculated. It is also the first step in
calculating the standard deviation.
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